The present invention relates to electric motor control, and more particularly to controlling electric motors during times when the electric power supply is unreliable.
Automotive vehicles incorporating a start/stop feature may rely on the main vehicle battery for electrical power during the engine off portion of the start/stop operations. Start/stop is a method of improving vehicle fuel economy by stopping the engine when the vehicle is halted for routine driving events (e.g., stopped at a traffic light). In start/stop, the engine control software stops the engine when the vehicle comes to a halt. The objective of stopping the engine is to improve fuel economy by not burning fuel when the vehicle is stopped and the engine is idling. Once the engine is halted, vehicle electrical devices are powered only by the battery (as opposed to the alternator that is used when the engine is on). When the driver releases the brake and steps on the accelerator, the engine is automatically restarted to allow the vehicle to continue on its way.
This engine restart causes what is called a low voltage event in the vehicle electrical system. When the engine starter motor first engages in order to restart the engine, it draws a large amount of current from the battery. This large current draw results in the battery voltage temporarily dropping to a low level that is several volts lower than it was prior to the engine restart. Even though the low voltage event typically lasts a few hundred milliseconds while the engine is cranking, the low battery voltage may cause many electrical devices in the vehicle to stop working because they no longer have sufficient electrical power to operate. After the engine restarts, voltage is then restored to the electrical system when the starter motor is turned off and the alternator begins operating again. Although, under certain operating conditions, when the power supply voltage to the motors in the vehicle is restored, the motors experience an in-rush of electrical current. If multiple motors in the vehicle simultaneously restart, they may (together) draw sufficient current to continue to disrupt the power supply voltage in the vehicle for a short period beyond the starter motor operation.
Electrical motors in the vehicle may be significantly affected by such low voltage events. When the motors are subjected to the low voltage associated with the start/stop event, the operation of the motors, and any operation they were in the middle of, may be affected. For example, a power window motor may have been in the process of opening or closing a window during the engine restart. During engine restart, the motors may momentarily stop working. Also, the motors may not “remember” what they were doing prior to the low voltage event. For example, motors in some subsystems (e.g., power windows) will not remember the direction in which they were operating (e.g., up or down) if power is interrupted or will lose track of their position within their range of movement. The management of electrical motors during start/stop events may have a significant impact on the customer perception of the start/stop feature.
For example, if this motor behavior is left uncorrected, the customer may experience vehicle functions that employ motors that halt during start/stop engine restarts, requiring the customer to manually restart the motor. Some features (e.g., power windows) may stop operation at some point in their range of travel, with the customer needing to perform a special operation to recalibrate the motor position. This motor behavior potentially leads to increased customer dissatisfaction with the vehicle start/stop function.
In addition, in general, this adverse motor behavior may increase as the battery ages, since aged batteries may be less able to support the electrical system load during the restart operations. As the customers notice that the behavior of some electrical features deteriorate over time, they may return the vehicles for service.
As a consequence of this motor behavior during start/stop operations, start/stop electrical systems attempt to employ hardware and software to mitigate the adverse behavior of motors during start/stop engine restart. One attempted solution is the use external electronic modules to control the motors. The external electronic modules basically perform two functions, monitor the operation of the motors and monitor the power supply voltage in the vehicle. When a low voltage event occurs and the motors are operating, the modules store information needed to remember the direction of motor travel and the location of the motor within the range of travel. When the low voltage event has passed, they reactivate the motors as is appropriate.
In some instances, the external electronic modules intercept customer requests to activate the motors (e.g., button presses). The modules then decide, based on vehicle status, whether the motors can operate without risking potential engine restarts. For example, if the battery state of health is low, in some circumstances the modules may request the engine to restart before the windows are allowed to move. This ensures that the in-rush current associated with activation of the windows will not coincide with the starter in-rush current, thereby assuring adequate battery voltage for a successful restart.
The difficulty with employing the external electronic modules is that they can be very costly to implement. For example, when external electronic modules are used for door control modules to control power windows for start/stop purposes, the cost of the modules is several tens of dollars. This increases the cost of including the start/stop feature in vehicles that would otherwise not need such modules. A second concern is that the implementation of these mitigating actions for motor control, discussed above, is that they may significantly increase the complexity of software and design validation testing for the vehicle.
A second attempted solution is to accept the degradation in motor behavior by not adding external modules specifically for motor control. In some instances it may not be practicable (for reasons of cost, complexity, package space, or other factors) to provide an external module for motor control. However, this attempted solution has drawbacks such as motors without external control posing a risk to successful engine restarts if their operation degrades the battery voltage during the restart. Moreover, the potential for such events may increase as the battery ages, the ambient temperature falls, or the vehicle electrical load (for other subsystems) increases.